The present invention relates to an image forming apparatus provided with a recording paper transport device having a plurality of independent driving systems. 2) Description of the Related Art
Recently, a paper feeding cabinet containing a plurality of paper cassettes independently accommodating recording papers of various sizes tends to be provided in an image forming apparatus, for example, in a copier. By providing such a paper feeding cabinet, the number of times of supplement or exchange of recording papers, which is troublesome, can be reduced, whereby operability of the copier is improved.
In a copier provided with this type of paper feeding cabinet, are employed two driving systems: a driving system comprising an image forming system including a photoconductive drum and various rollers for transporting a recording paper in a main body; and a driving system for feeding the recording paper from the paper cassette to the main body.
In the former, the rollers include a pair of timing rollers for transporting the recording paper to a transfer charger disposed around the photoconductive drum in synchronism with the rotation of the photoconductive drum and a pair of intermediate rollers for transporting the recording paper to the timing rollers. Each of the above two pairs of rollers is driven by a main motor for driving the photoconductive drum through a clutch.
On the other hand, the latter comprises a paper feeding roller for feeding a recording paper from the paper cassette, a pair of separating rollers for separating one recording paper from the others for feeding in case that a plurality of recording papers are fed by the paper feeding roller and a pair of transport rollers for transporting the separated recording paper to the intermediate rollers. The above rollers are driven by a paper feeding cabinet driving motor (referred to as driving motor, hereinafter) provided in the paper feeding cabinet through a gear mechanism.
In the former, the moment of inertia of a portion which is stopped and driven along with the intermediate rollers is relatively small since the rollers are driven and stopped by engaging and disengaging the clutches (turning on and turning off the clutches), while in the latter, the moment of inertia of a portion which is stopped and driven is large since the transport rollers are driven and stopped by turning on and off the driving motor M2.
This difference in the moment of inertia gives rise to the problems which will be described below when the leading and trailing portions of the recording paper are respectively positioned at the above two driving systems, more specifically, when the recording paper is stopped with its leading portion and trailing portion being held respectively by the intermediate rollers and the transport rollers and when the recording paper which has been stopped in the above condition is transported again by driving the transport rollers and intermediate rollers.
FIG. 1a is a time chart showing reduction in the circumferential speeds of the transport rollers and the intermediate rollers when stopping. In FIG. 1a, the axis of ordinate indicates the speeds and the axis of abscissa indicates time.
As shown in FIG. 1a, when the clutch between the intermediate rollers and the main motor and the driving motor are turned off at the same time, the intermediate rollers are stopped earlier than the transport rollers due to the above difference in the moment of inertia. Therefore, the trailing portion of the recording paper is transported by the transport rollers by a distance indicated by oblique lines in the drawing with its leading portion stopped between the intermediate rollers. As a result, the recording paper is curved or is folded in the middle portion thereof, resulting in deterioration of the quality of the recording paper or paper jamming.
FIG. 1b is a time chart showing starting points of the rotations of the transport rollers and the intermediate rollers. In FIG. 1b, the axis of ordinate indicates the speeds of the above rollers and the axis of abscissa indicates time. As shown in FIG. 1b, where the intermediate rollers and the transport rollers which have been stopped, respectively holding the leading portion and trailing portion of the recording paper, are driven by turning on the clutch and the driving motor at the same time, the intermediate rollers start rotating instantly, increasing the rotating speed rapidly, while the transport rollers start rotating after the lapse of a dead time t0. As a result, the leading portion of the recording paper is rapidly drawn forward by the intermediate rollers. This applies great tension to the recording paper, thereby deteriorating the quality of the recording paper. In addition, the recording paper may be skewed.
The latter problem can be relieved by providing a one-way clutch for allowing the transport rollers to rotate faster than the rotating speed of the driving motor in the same direction as a transport direction of the recording paper. However, in this case also, great tension is applied to the recording paper at the instant the intermediate rollers start rotating.
The above problems are caused when the length of the recording paper in the transport direction (referred to as length Y, hereinafter) is greater than a distance X between the timing rollers and the transport rollers (the sum of a distance A between the timing rollers 22 and the intermediate rollers 23 and a distance B between the intermediate rollers 23 and the transport rollers 205 in FIG. 2). This is for the following reason.
The recording paper is stopped when the leading end thereof has reached the timing rollers 22. Therefore, where the length Y is X or more, the leading and trailing portions of the stopped recording paper are respectively held between the intermediate rollers 23 and between the transport rollers 205.
Further, even if the length Y is less than X, there is the following problem in a continuous copying mode. That is, if the clutch and the driving motor are turned off at the same time, a preceding recording paper transported by the intermediate rollers 23 is stopped earlier than the following recording paper transported by the transport rollers 205. As a result, the preceding recording paper may be bumped by the following recording paper, which also causes paper jamming. Therefore, distance between the recording papers should be long enough to avoid such a problem. This prevents paper feeding efficiency from being improved and restricts the speed of the continuous copying.
In addition, where the plurality of paper cassettes contained in the paper feeding cabinet are driven by the same driving motor, driving load of the driving motor and other elements are varied with paper feeding conditions, for example, the number of paper cassettes (referred to as cassette number, hereinafter). Therefore, if a signal for turning off the driving motor is output at a fixed timing regardless of the paper feeding conditions, stopping point of the driving motor, in other words, stopping point of the transport rollers 205 is varied with the above conditions.
More specifically, the more the cassette number is, the greater the driving load is, that is to say, the earlier the driving motor is stopped.
The paper feeding conditions include a size of a recording paper to be fed (the larger the size is, the greater the driving load is), thickness of the recording paper (the greater the thickness is, the greater the driving load is, due to increase in the recording paper weight and in travel resistance of the recording paper between each pair of rollers), transport speed of a recording paper (transport speed: the rotating speed of the motor; the greater the number is, the longer time the motor takes to stop), paper feeding position (position of the paper cassette from which the recording paper is fed; the farther the paper cassette is from the main body, the greater the driving load is, for example, due to increase in the length of a belt for transmitting driving force), inside temperature of the apparatus and fluctuation of the travel resistance of the recording paper accompanying humidity change in the apparatus.
FIG. 3a is a time chart showing a relationship between the driving load and the stopping point of the driving motor. In FIG. 3a, the axis of ordinate indicates the speed of the driving motor and the axis of abscissa indicates time. FIG. 3b is a view showing a relationship between the paper feeding conditions and the driving load of the driving motor.
As shown in FIG. 3a, the greater the driving load is, the shorter the time from turn-off to stop of the driving motor is (t4&lt;t3&lt;t2&lt;t1).
Accordingly, in order to prevent paper jamming and improve copying operation efficiency, the stopping point of the driving motor should be determined taking the paper feeding conditions into account.